Silicon based semiconductor devices, such as integrated circuits, typically include a silicon dioxide (SiO.sub.2) dielectric layer. Multilevel circuit traces, typically formed from aluminum or an aluminum alloy, are patterned onto the Sio.sub.2 substrate.
If the aluminum based circuit traces are replaced with copper based circuit traces, the density of circuit traces on the face of the device could be increased because copper has a higher electrical conductivity than aluminum enabling the use of circuit traces with a reduced cross-sectional area. In addition, the electromigration of copper is approximately 0.1 that of aluminum at a given temperature.
In order to use multilevel metals circuit traces, lithographic focus constraints require that the substrate surface be planar on both a global and local scale. If the surface is not planar, the exposure tool can not be focused properly resulting in out of focus images and poor quality printing.
One way to fabricate planar copper circuit traces on a silicon dioxide substrate is referred to as the damascene process. In accordance with this process, the silicon dioxide dielectric surface is patterned by a conventional dry etch process to form holes and trenches for vertical and horizontal interconnects. The patterned surface is coated with an adhesion-promoting layer such as titanium or tantalum and/or a diffusion barrier layer such as titanium nitride or tantalum nitride. The adhesion-promoting layer and/or the diffusion barrier layer is then overcoated with copper. Chemical-mechanical polishing is next employed to reduce the thickness of the copper overlayer, as well as the thicknesses of any adhesion-promoting layer and/or diffusion barrier layer, until a planar surface that exposes elevated portions of the silicon dioxide surface is obtained. The vias and trenches remain filled with electrically conductive copper forming the circuit interconnects.
Previously, it was believed that the removal rate of the copper and the adhesion-promoting layer and/or the diffusion barrier layer must both greatly exceed the removal rate of silicon dioxide so that polishing effectively stops when elevated portions of the silicon dioxide are exposed. The ratio of the removal rate of copper to the removal rate of silicon dioxide base is called "selectivity." A minimum selectivity of about 50 was desired for chemical-mechanical polishing. However, when high selectivity copper slurries are used, the copper layers are easily over-polished creating a depression or "dishing" effect in the copper vias and trenches. This feature distortion is unacceptable due to lithographic and other constraints in semiconductor manufacturing.
Another feature distortion that is unsuitable for semiconductor manufacturing is called "erosion". Erosion is the topography difference between a field of silicon oxide and a dense array of copper vias or trenches. In chemical-mechanical polishing, the materials in the dense array are removed or eroded at a faster rate than the surrounding field of silicon oxide. This causes a topography difference between the field of silicon oxide and the dense copper array. The industry standard for erosion is typically less than 500 Angstroms (.ANG.).
Chemical-mechanical polishing has two components, a chemical component and a mechanical component. An important consideration in slurry selection is "passive etch rate." The passive etch rate is the rate at which copper is dissolved by the chemical component alone and should be significantly lower than the removal rate when both the chemical component and the mechanical component are involved to prevent under-cutting of copper contained in the trenches and vias by contact with the chemical component. A large passive etch rate leads to dishing and thus, preferably, it is less than 10 nanometers per minute.
A number of systems for chemical-mechanical polishing of copper have been disclosed. Kumar et al. in an article entitled "Chemical-Mechanical Polishing of Copper in Glycerol Based Slurries" (Materials Research Society Symposium Proceedings, 1996) disclose a slurry that contains glycerol and abrasive alumina particles. An article by Gutmann et al. entitled "Chemical-Mechanical Polishing of Copper with Oxide and Polymer Interlevel Dielectrics" (Thin Solid Films, 1995) discloses slurries based on either ammonium hydroxide or nitric acid that may contain benzotriazole (BTA) as an inhibitor of copper dissolution. Luo et al. in an article entitled "Stabilization of Alumina Slurry for Chemical-Mechanical Polishing of Copper" (Langmuir, 1996) discloses alumina-ferric nitrate slurries that contain polymeric surfactants and BTA. Carpio et al. in an article entitled "Initial Study on Copper CMP Slurry Chemistries" (Thin Solid Films, 1995) disclose slurries that contain either alumina or silica particles, nitric acid or ammonium hydroxide, with hydrogen peroxide or potassium permanganate as an oxidizer.
There are a number of theories as to the mechanism for chemical-mechanical polishing of copper. An article by Zeidler et al. (Microelectronic Engineering, 1997) proposes that the chemical component forms a passivation layer on the copper changing the copper to a copper oxide. The copper oxide has different mechanical properties, such as density and hardness, than metallic copper and passivation changes the polishing rate of the abrasive portion. The above article by Gutmann et al. discloses that the mechanical component abrades elevated portions of copper and the chemical component then dissolves the abraded material. The chemical component also passivates recessed copper areas minimizing dissolution of those portions.
While present day chemical-mechanical polishing systems are capable of removing a copper overlayer from a silicon dioxide substrate, the systems do not satisfy the rigorous demands of the semiconductor industry. These requirements can be summarized as follows. First, there is a need for high removal rates of copper to satisfy throughput demands. Secondly, there must be excellent topography uniformity across the substrate. Finally, the CMP method must minimize local dishing and erosion effects to satisfy ever increasing lithographic demands.
Accordingly it is an object of the present invention to provide slurries that rapidly remove copper. It is also an object of the present invention to provide a slurry that will reduce the amount of dishing in copper vias and trenches. It is a further object of the present invention to provide a method that will rapidly polish copper layers without causing dishing and erosion effects.